Method of producing thin microporous silica coatings having reflection reducing characteristics and the articles so coated



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SEARCH ROOM June 21, 1949. MOULTON 2 474,061

METHOD OF PRODUCING THIN MICRO-POROUS S IL CA COATINGS HAVING REFLECTIONREDUCING CHARACTER STIC AND THE ARTICLES 50 COATED Filed July 23, 1943INVENTOR. HAROLD l2. MOULTON AT TOIZNEY Patented June 21, 1949 SEARCHROOM METHOD OF PRODUCING THIN MICROPOR- OUS SILICA COATINGS HAVINGREFLEC- TION REDUCING CHARACTERISTICS AND THE ARTICLES SO COATED HaroldR. Moulton, Southbridge, Mass, assignor to American Optical Company,Southbridge, Mass., a voluntary association of Massachusetts ApplicationJuly 23, 1943, Serial N0. 495,906

14 Claims.

This invention relates to new and novel means and methods of reducingsurface reflections and, in some instances, producing surfaces withincreased scratch resistance and of producing articles having suchsurfaces thereon.

This application is a continuation in part of my copending applicationSerial No. 452,206, filed July 24, 1942, now abandoned.

One of the principal objects of the invention is to provide novel meansand methods of reducing the reflection of light impinging upon thesurface of an article whereby, in the case of a transparent article, agreater percentage of incident light will traverse said article and inthe case of an opaque article will reduce the reflectivity of saidarticles, and in some instances simultaneously increasing the scratchresistance of the surfaces of said articles.

Another object is to provide simple, efiicient and economical methods ofproviding articles with coatings having the above characteristics.

Another one of the principal objects of this invention is to obtaincoatings of the above character which are inherently free from haze andwhich remain so throughout the useful life of the article on which thiscoating is applied.

Another object is to provide new and improved methods whereby coatingsof the above character may be applied to the completed articles.

Another object is to provide new and improved means and methods offorming stable color screens and optical fllters.

Another object is to provide new and improved diffusing screens andmeans and method of forming the same.

Another object is to provide novel means and changes may be made in thedetails of construction, arrangement of parts and steps of the meth odsshown and described without departing from the spirit of the inventionas expressed in the accompanying claims. I, therefore, do not wish 3 tobe limited to the exact details of construction, arrangement of partsand methods shown and described as the preferred forms only have beengiven by way of illustration.

therein illustrates an article having a coating thereon formed accordingto the invention.

In the prior art, surface coatings have been produced for reducingreflections of light impinging upon the surfaces by commonly knownmethods.

A method which has retained a considerable commercial application is anoutgrowth of vacuum evaporation technique. In this method, a thin layer,generally one quarter of a wave length thick, of a volatile salt orcompound of low index is evaporated by means of heat in a vacuum and adeposit is produced upon the surface to be treated. The compounds usedare customarily lithium fluoride, calcium fluoride, cryolite, ormagnesium fluoride or similar compounds. These compounds produce surfacelayers which, when very carefully controlled as to thickness, willbecome quite eflicient as reflection reduction coatings. As such, theyhave been used in multicomponent lens systems for expensive opticalparts. It has been found necessary, in order to increase the resistanceto atmospheric attack and to the attack of moisture to bake thesecoatings at a relatively high temperature.

Another prior art method of obtaining glass surfaces having reducedreflection consists in the treatment of the surfaces with hydrofluoricacid gas or with dilute aqueous solution of hydroents of the glass arerendered soluble or volatile and are then subsequently abstracted fromthe Referring to the drawing, the figure shown 55 surface leaving a fllmof lower index. Other selected solvents for certain of the compoundspresent in the glass may be used and have been used in the prior art,such as alkalis, soluble phosphates, nitric acid and even harsh laundrysoap. The use of the above described process, of course, is restrictedto glass.

It, therefore, is one-of the primary objects of this invention toproduce surface coatings which distinguish from the above in that theydo not require high vacuum equipment or prolonged treatment, which maybe applied in a simpler, more convenient and inexpensive manner and atthe same time with extreme resistance to th attack of moist atmosphereand moisture itself, and in some instances more resistant to scratchingand chemical attack, and which in addition may be applied to materialsother than glass.

In addition to the reduction of reflection of incident light thecoatings as produced serve as chemical protection for the glass orplastic sur-' faces.

In following the teachings of the invention the article I which is to becoated may be of a light transmitting nature such as glass plates,lenses, prisms or other optical elements and similar elements formed ofplastics or artificial resins. In this particular instance, the element,as stated above, is to have light transmitting properties.

In the present instance, the articles I referred to are particularly ofthe type which are adapted for use in telescope systems, field glasses,binoculars, camera lenses, periscopes or for any other uses in which thelight is adapted to impinge upon a surface of the article. The surfaces2 of said articles, in order to reduce surface reflection and toincrease light transmission, are coated with a stable water insolublecoating 3 of a material having a lower index of refraction than thesubstrate. This is accomplished by applying a layer of the solution,which upon evaporation, deposits such a layer on the glass or materialsubstrate. The material which forms this layer need not be present inthe solution in the actual form in which it finally exists on thesurface of the article but may be a decomposable compound which upon theremoval of the solvent leaves a surface layer on the substrate in thedesired state, for example, a solution of from 1 to 10 per cent byvolume of tetraethylorthosilicate, to 60 per cent by volume ethylacetate, 1 to 10 per cent by volume concentrated hydrochloric acid andthe balance ethyl alcohol. Denatured alcohol may be substituted for theethyl alcohol. A preferred solution is five parts oftetraethylorthosilicate, 50 parts of ethyl acetate, 50 parts ofdenatured alcohol and 5 parts of concentrated hydrochloric acid, mixedin the order given. This gives a clear, colorless, stable solution. Anessential feature of this solution is aging for several days before use.The exact reason for this is not known but it is believed to be apartial decomposition of the tetraethylorthosilicate by the hydrochloricacid resulting in an organo-sol of either hydrated or unhydrated silica.With the composition set forth above, an aging of from two to six daysis desirable. It has been found desirable for certain articles, such aslenses, especially those of steep curve, to spin the article atrelatively slow speed and pour upon the upper surface thereof a smallmeasured quantity of the solution. In this way, one surface may be doneat a time and the smoothness and uniformity of the coating may beincreased. In either case, the spinnin of the article is continued untilthe layer of solution formed thereby has evaporated sufiiciently so thatno drips or runs will occur when the article is removed from the holder.Prolonged aging to two or three months produces no further change. Thisaging normally takes place at room temperatures but it can beaccelerated slightly by working at more elevated temperatures up to 50C.

The solution having been prepared and aged, the procedure is as follows:

The lens or other article is immersed in this liquid, removed andimmediately spun in order to remove excess liquid. Over a very widerange of translational velocities the effective thickness of theresulting layer is in the range for emclent reflection reduction therebydiffering from previous methods of obtaining reflection reduction inwhich the efficiency of the final product is greatly dependent upon thevariation of thickness in the film. At this point, at least threealternative methods may be as follows:

In method 1 the article is placed in a moist atmosphere at a temperatureof approximately C. for a time interval suflicient to substantially heatthe entire article to said temperature after which the article may becooled. This procedure renders the coating insoluble in acids or waterand difilcultly soluble in alkaline solutions. The coating produced bythis method reflects approximately 1% on glass of 1.52 index of lightfrom each surface. In other words, for a flat piece of glass treated bythis method substantially 98% of the incident light traverses it incontradistinction to the figure 91.8 which is the inherent transmissionof glass of this index of refraction. If the glass, for example, is of ahigher index of refraction than 1.52 the reduction of reflection isstill greater. The treatment at 100 C. moist heat, serves to expeditethe rendering insoluble of the surface reflection reducing coating.There is a more rapid increase in the hardness of this surface thanoccurs in the next two methods but the end result in all cases issubstantially the same as regards hardness and insolubility. Although Ihave referred to the temperature as being 100 C. it is to be understoodthat any suitable heating temperatures may be used and been found that awider range of temperatures may be used and produce surface coatings ofsubstantially similar characteristics, the top limit of suchtemperatures being set by the softening or distortion point of thematerial bein treated, the lower temperature of 100 C. being preferablefor use as it permits composite structures which are united by cementingor the like which are capable of withstanding this temperature but whichwould be susceptible to injury at higher temperatures. Lowertemperatures, which are above room temperatures, are thereforepreferably desirable. The temperature, therefore, must mainly be such asto produce coatings having desired characteristics without injury to theinitial article. The temperatures referred to hereinafter are thereforeto be considered in the light of the above.

The second method consists in holding the article at room temperatureafter the solution has been applied and the excess removed by anyconvenient means such as the spinning as set forth above. The article isheld at room temperature in normally moist air for a period ofapproximately one minute, the time varying in-' versely with the ambienttemperature. At the expiration of this exposure period the article issubjected to the action of water which may be cool or warm for anyconvenient period of time as it has been found that variations in thetime of water treatment have substantially the same effect. Thistreatment somewhat increases the reflection reduction above mentionedbut the article may be heated to 100 C. subsequently in order to insurepermanency. It is felt that this gives a somewhat more porous surfaceand one which is somewhat less easy to clean.

The third method consists in simply allowing the article to stand atroom temperature after the excess solution has been drained off orremoved by spinning in which case the article should not be handled forsome little time. About an hour has been found a desirable period.

For coating larger surfaces, it has been found that dflution of theoriginal solution with a suitable solvent of which a mixture of equalparts of denatured alcohol and ethyl acetate have been found to producepractical results, may be used. In this case, multiple dips followed bydraining, are desirable, with the article being so oriented SEARCH ROGas to drain in a different direction after each dip whereby theeffectiveness of the coating is maintained substantially uniform overthe article. This procedure is particularly useful in coating largesheets.

Each coating may be heat treated, as above described, before theapplication of the succeeding coating or the coating may be simplyallowed to dry in air as described in method #3 or the final product maybe treated with water as described in method #2 or any combination ofthese procedures may be followed either on each individual coating or onany one of the successive coatings.

In the above example, tetraethylorthosilicate has been mentioned as hashydrochloric acid. denatured alcohol and ethyl acetate. It is, ofcourse, understood that other alkyl silicates, such astetramethylorthosilicate, or other esters of sillcic acid may be usedinstead of tetraethylorthosilicate and it is also to be understood thatother acids instead of hydrochloric acid may be used with equaleffectiveness such as nitric acid, sulphuric acid, hydrobromic acid, andthe solvents used, denatured alcohol and ethyl acetate, have beenselected as being the convenient ones readily commercially available andcheap. Other solvents, however, it has been found, may be used. The solesolvent may be denatured alcohol, for example, but it is found that theethyl acetate assists in giving a uniform layer. In this particularinstance, for example, methyl acetate, methyl alcohol, amyl acetate,isopropylalcohol, or in fact most organic solvents may be used in whichboth the silicon ester and the acid are both soluble. It is to beunderstood also that the organo-sol of silica may be separately preparedand purified and used in suitable solvents.

It has been found that other organic solvents than those enumerated maybe used to advantage; for example:

Ethylene glycol mono ethyl ether approximately 24%; ethylene glycol monobutyl ether approximately 10%, butyl alcohol approximately 16%,denatured alcohol approximately 40%, tetraethylorthosilicateapproximately 5%, and concentrated hydrochloric acid approximately Therelative proportions of these various ingredients may be varied inaccordance with the method of application as lower speed of rotationduring or subsequent to the application to the article to be coated willrequire a lower concentration of the active ingredient; namely thetetraethylorthosilicate. The high boiling solvents, such as ethyleneglycol mono ethyl ether, ethylene glycol mono butyl ether and butylalcohol may have their proportions varied so as to control the rate ofevaporation of the solvent. In the case of high atmospheric humidity areduction in the amount of the denatured alcohol and its replacement byhigher boiling solvents results in a more uniform haze-free coating.

Another composition which may be used advantageously consists ofethylene glycol mono ethyl ether approximately 94% by volume,tetraethylorthosilicate approximately 5% by volume, concentratedhydrochloric acid approximately 1% by volume. This particularcomposition is particularly resistant to the effects of atmospherichumidity. It has been found also that a composition consisting ofapproximately 72% denatured alcohol, 7% tetraethylorthosilicateapproximately 20% ethylene glycol monoethyl ether and approximately 1%hydrochloric acid gives excellent coatings of high permanence and goodefficiency. Substantially 50% of the ethyl alcohol may be replaced byethyl acetate if desired.

It is believed that there has been produced a 5 micro-porous insolubleabrasion resistant layer of silicon dioxide on the surface of thearticle, the pores being smaller in dimension than a wave length oflight. This micro-porous silica layer has an effective index ofrefraction due to its porosity of less than 1.4 thus more nearlyfulfilling the requirements for a layer whose index of refraction isequal to the square root of the index of refraction of the substratethan do silicon dioxide layers produced by evaporation and the like. Therelative proportions under the conditions of use are such as to producea film of this index and of such thickness that the reflected light fromone surface of the layer is approximately one half wave out of phasewith the reflected wave from the other surface 0f the layer, or anynumber of full waves plus one half wave out of phase.

It is to be understood also that there may be incorporated in thesolution substances which retain their solubility in suitable solventsafter the body of the film itself has been rendered insoluble. Suchsubstances when subsequently leached out by suitable solvent treatmentmodify the porosity of the surface reflection reducing layer and give anadditional control of the resultant effective index of refraction ofsaid layer. It has been found that substances, such as urea,paraphenylenediaminedihydrochloride, glycerine, caffein hydrochloride,magnesium chloride, calcium chloride and in general other compoundssoluble in the solvents used in making up the solution and which remainuniformly and minutely dispersed throughout the resulting coating andwhich after the coating itself has become set and insoluble maysubsequently be removed by a solvent such as water leaving, when leachedout, a coating which is considerably more microporous than the coatingwhich does not contain the soluble constituents. This gives anadditional reduction of the refractive index of the resulting finishedcoating.

While these coatings in their finished form are of great stability,before the heating operation or before the coatings have stood too longit is possible to remove a coating which has been damaged or which forsome reason was defective, by treatment with dilute alkali in whichinstance the article can be recoated without damage.

.In the solution there is a compound which is capable of beingdecomposed and depositing a layer of silica, such a compound beingtetraethylorthosilicate or other analogous compounds and also in thesolution there is a compound capable of decomposing the decomposablesilicon compounds, this decomposing agent being the acid referred toabove. In the dilute solution, as applied, it is believed that the tworeacting materials do not interact rapidly or completely but that as thesolvent is removed the concentrations of the decomposable substance andthe decomposing agent build up until concentrations are reached at whichthe reaction takes place rapidly. At this time and not before, thecoherent film or layer of silica is deposited on the surface of thearticle to which the solution has been applied, the film subsequentlybeing hardenedeither by aging or by treatment with moist heat asdescribed above.

In the examples given, hydrochloric acid is the 75 decomposing agentwhich reacts with tetraethyl- .applied in a single surface. coating.

acre-eel orthosillcate to form silicon dioxide. If these two ingredientsare mixed in concentrated form the mixture becomes hot and solidifies ina few minutes. The solvent or solvents used in the disclosed solutionsserve to prevent jelling and produce a relatively permanent solutionuseable for a period of time and one which has suitable evaporationcharacteristics. The solvent used may be, of course, simple or complex,that is to say, it may consist of a single chemical compound, such asethyl acetate, alcohol or in fact any solvent in which the reactingsubstances can be dissolved. The solvent may be a complex solventconsisting of two or more suitable solvents mixed in any desiredproportion within the range of workability in which case the relativeevaporation rates of the solvents may differ and the process of buildingup the concentrations of the decomposable substance and thedecomposition agent may be further controlled.

When surface coatings of the type set forth herein are applied tomaterials such as polymeric methyl methacrylate and the like, celluloseacetate, cellulose nitrate, cellulose acetate-butyrate,

polystyrene and other resinous or plastic materials the coating has aconsiderably higher intrinsic hardness than the inherent surfacehardiness of the article itself and therefore imparts an increasedresistance to scratching and abrasion. In addition, the inert chemicalnature of the resulting coating and its freedom from cracks, and

ing which is a superficial cracking of the surface of a stressed plasticpart by exposure to vapors of a solvent or to the solvent itself.

Although we have specified that the articles to be treated are of atransparent nature it is to be understood that any article of opaque ortransparent material might be similarly coated; for example, an articleof suitable metal might be provided with surface coatings of the abovecharacter in which instance the article will be rendered less visible ormore resistant to chemical injury and, of course, the same statementsapply to opaque glasses, glazes, paints, plastics or in fact anysubstance crystalline or non-crystalline.

Although thegiven proportions are set forth above, it is to beunderstood" that the proportions of the various ingredients might bevaried in accordance with the procedure followed and in order to producea thicker coating the content of the silica ester would be increased andto produce tially increased or decreased proportionately. Theconcentration of the various ingredients varies according to the numberof surface coatconcentrations may be formed relatively weak as comparedwith concentrations which are to be .65 thinner coatings the proportionswould be de- "creased with the remaining ingredients substan- .ingsapplied. If a plurality of surface coatings ,are to be applied, it is tobe understood that the 8 The coating compositions set forth above do notdepend upon fortuitous decomposition of the decomposable siliconcompounds by atmospheric moisture or by moisture or other substancesdiifusing out of the material to which the coating is applied, but thedecomposition agent is present in the solution in controlledconcentration and in such a form that until the coating has beenconcentrated by evaporation after application to the article thereactions do not take place and the solution is completely stable untilsuch time as this evaporation takes place. This is one of the maindistinctions of the present invention over known prior art.

The siliceous coatin formed by the above described methods may alsoserve as the carrier and/or protector of coloring matters, such as dyes,pigments and the like. In general, spirit soluble dyes should be used insuitable quantities to produce the desired tinctorial effect in thefinished film and may be dissolved in the solution used in forming suchfilm. Upon evaporation of the solvent and the completion of thehardening of the film the dyestufi or pigment remains in permanent formuniformly dissolved in or scat.- tered throughout the film. Likewise,white finely divided pigments, such as titanium dioxide or other similarwhite pigments having a high index of refraction may be suspended in thefilm-form'- ing solution and upon the removal of the solvent andcompletion of the formation of such film the particles remain thereinand serve as light diffusing means. The film containing either the dyesor the pig'- ments may also be used for modifying the color of articlesin general, mineral, metallic, or organic, whereby a surface color ofhigh permanence may be applied to the article.

Incorporation of fluorescent compounds, either in solution or as finelydivided particles in the coating solution, produce fluorescence whenirradiated with radiation of suitable wave length. 'In the prior art,such fluorescent pigments have made use of organic binders, such aslacquers, ,varnishes, etc. which suffer from the serious drawback oflimited transparency to the radiation and lack of heat stability and/orchemical stability. The siliceous coating having these fluorescentsubstances incorporated in it is "obviously free from these drawbackshaving "great heat stability and high chemical permanency.

Certain inorganic compounds, such as cerium compounds, possessing theproperty of absorbing ultra-violet radiation may be incorporated in theoriginal solution and upon evaporation of the solution remain in thesiliceous layer.

Certain organic compounds, not generally con- "sidered as dyestuffs,such as diphenyl, quinin, anthracene and naphthalene, for example, maybe dissolved in the original solution and produce 'upon evaporation ofthe solvents films havin ultra-violet absorption.

Coatings of the above character are particularly advantageous when usedon articles having relatively smooth or highly polished optical sur--faces such, for example, as exist on lenses or other optical elements.In the case of lenses 'which have highly polished optical surfacesthereon such surface coatings increase the trans.- mission of light andthereby greatly increase the efficiencyv of said optical elements.

It is to be understood that when applicant prefers, to acids throughoutthe specification and SEARCH Rom:

in the claims as a general statement, it is intended to mean any acidwith the exception of hydrofluoric acid, or to mean an acid which isinert as to its effect on silica.

The compositions of the solutions render it possible to apply saidcoatings with great ease and with great uniformity and speed without therequirement of special coating equipment and further distinguishes frommost prior art coating technique in that the solutions are particularlyadaptable to wide area coverage with ease and simplicity and areexceedingly transparent and particularly adaptable for coating opticalelements because of the uniformity of coating which may be obtained withsuch composition and which will have little or no altering effect uponthe desired refractive characteristics of said optical elements. Inaddition, the formation of such a silica film or layer is not dependentupon the chance occurrence in the atmosphere of a decomposing agentwhich, of course, is not controllable and produces inconsistent results.The solutions of the present invention positively control thedecomposing characteristics and provide means for obtaining positive,accurate and rapid results under definite control.

From the foregoing description it will be seen that simple, eflicientand economical means and methods have been provided for acomplishing allof the objects and advantages of the invention.

Having described my invention, I claim:

1. An article of the character described comprising a substrate having athin transparent surface reflection reducing coating of substantiallypure silica formed on the surface thereof, said coating beingmicroporous and consisting of the dried silica product of atetra-alkylorthosilicate selected from the group consisting oftetra-ethyl-orthosilicate, tetra-methyl-orthosilicate and mixturesthereof hydrolyzed in situ by an acid inert to silica and the substrate,said coating having a substantially uniform optical thickness of an oddmultiple of a quarter wave length of the incident light and asubstantially uniform effective index of refraction less than that ofmassive silica, said coating being firmly adherent to the substrate andbeing resistant to abrasion.

2. An article of the character described comprising a piece of lighttransmitting material having a thin transparent surface reflectionreducing coating of substantially pure silica formed on asurfacethereof, said coating being microporous and consisting of the driedsilica product of tetra-ethyl-orthosilicate hydrolyzed in situ by anacid inert to silica and the light transmitting material, said coatinghaving a substantially uniform optical thickness of an odd multiple of aquarter wave length of the incident light and a substantially uniformeffective index of refraction less than that of massive silica, saidcoating being firmly adherent to the piece of light transmittingmaterial and being resistant to abrasion.

3. An article of the character described comprising a substrate having athin transparent surface reflection reducing coating formed on a surfacethereof, said coating being microporous and consisting of the driedsilica product of a tetra-alkyl-orthosilicate selected from the groupconsisting of tetraethylorthosilicate, tetramethyl-orthosilicate andmixtures thereof hydrolyzed in situ by an acid inert to silica and thesubstrate. and containing dispersed there- 10 through inert substancespossessing light transmission altering characteristics other than thatpossessed by the dried silica product, said coating having asubstantially uniform optical thickness of an odd multiple of a quarterwave length of the incident light and a substantially uniform effectiveindex of refraction less than that of massive silica, said coating beingfirmly adherent to the substrate and being resistant to abrasion.

4. The method of producing a surface refiection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of controlled amounts oftetra-alkyl-orthosilicate selected from the group consisting oftetraethylorthosilicate, tetramethylorthosilicate and mixtures thereofand a mineral acid inert to silica as a decomposing agent for thetetra-alkyl-orthosilicate, the tetra-alkyl-orthosilicate beingdecomposable by said acid to produce silica, saidtetra-alkyl-orthosilicate and acid being dissolved in a volatile organicsolvent miscible with both, said solvent being selected from the groupconsisting of mono ethyl ether of ethylene glycol, mono butyl ether ofethylene glycol, ethyl acetate, methyl acetate, amyl acetate, ethylalcohol, methyl alcohol, isopropyl alcohol, butyl alcohol, and mixturesthereof, the proportion of acid embodying approximately 1 to 10% of thesolution, the combined proportions of acid and solvent embodying atleast of the solution, and the proportion of tetra-alkyl-orthosilicateembodying substantially the balance of the solution, thetetra-alkyl-orthosilicate being held in the solution in a substantiallyundecomposed state until the concentration of thetetra-alkylorthosilicate and the acid is increased by evaporation of thesolvent sufliciently to permit the acidimdecomposeihe tetra-alkylorthosillcate, causing the liquid solutiontoform on said surface of thearticle a liquid layer of a uniform thickness which will contain anamount of tetra-alkyl-orthosilicate and acid sufiicient to produce ondecomposition of the tetra-alkyiorthosilicate a. coating of silicahaving an optical thickness of an odd multiple of a quarter wave lengthof the incident light, and evaporating the solvent from said liquidlayer so as to bring about the decomposition of thetetra-alkylorthosilicate by the acid on the surface of the article and.the production of the silica coating in situ on said surface and to thedesired optical thickness.

5. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of controlled amounts oftetra-ethyl-orthosilicate and a mineral acid inert to silica as adecomposing agent for the tetra-ethyl-orthosilicate, thetetra-ethyl-orthosilicate being decomposable by said acid to producesilica, said tetra-ethyl-orthosilicate and acid being dissolved in avolatile organic solvent miscible with both, said solvent being selectedfrom the group consisting of mono ethyl ether of ethylene glycol, monobutyl ether of ethylene glycol, ethyl acetate, methyl acetate, amylacetate, ethyl alcohol, methyl alcohol, isopropyl alcohol, butylalcohol, and mixtures thereof, the proportion of acid embodyingapproximately 1 to 10% of the solution, the proportion oftetraethyl-orthosilicate embodying approximately 1 to 10% of thesolution, and the proportion of solvent embodying substantially thebalance of the solution, the tetra-ethyl-orthosilicate being held in thesolution in a substantially undecomposed state until the concentrationof the tetraethyl-orthosilicate and the acid is increased by evaporationof the solvent sufficiently to permit the acid to decompose thetetra-ethyl-orthosilicate, causing the liquid solution to form on saidsurface of the article a liquid layer of a uniform thickness which willcontain an amount of tetra-ethyl-orthosilicate and acid sufficient toproduce on decomposition of the tetra-ethylorthosilicate a coating ofsilica having an optical thickness of an odd multiple of a quarter Wavelength of the incident light, evaporating the solvent from said liquidlayer so as to bring about the decomposition of thetetra-ethyl-orthosilicate by the acid on the surface of the article andthe production of the silica coating in situ on said surface and to thedesired optical thickness, subjecting the coated article to a moistatmosphere of approximately 100 C. for a period of time sufficient toheat the coated article to said temperature, and then cooling.

6. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of controlled amounts oftetra-alkyl-orthosilicate selected from the group consisting oftetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof,a mineral acid inert to silica as a decomposing agent for thetetra-alkyl-orthosilicate and a substantially involatile water solublesubstance inert in the presence of the acid, thetetra-alkylorthosilicate, the solvent to be used and the material of thearticle, the tetra-alkyl-orthosilicate being decomposable by said acidto produce silica, said tetra-alkyl-orthosilicate, acid and watersoluble substance being dissolved in a volatile organic solvent misciblewith each, said solvent being of the group selected from mono ethylether of ethylene glycol, mono butyl ether of ethylene glycol, ethylacetate, methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol,isopropyl alcohol, butyl alcohol, and mixtures thereof, the proportionof acid embodying approximately 1 to of the solution, the combinedproportions of acid and solvent embodying at least 90% of the solution,and the proportion of tetra-alkyl-orthosilicate embodying approximately1 to 10% of the solution, and the water soluble substance the balance ofthe solution, the tetra-alkyl-orthosilicate being held in the solutionin a substantially undecomposed state until the concentration of thetetra-alkyl-orthosilicate and the acid is increased by evaporation ofthe solvent sufiiciently to permit the acid to decompose thetetra-alkyl-orthosilicate, causing the liquid solution to form on saidsurface of the article a liquid layer of a uniform thickness which willcontain an amount of tetra-alkylorthosilicate and acid sufficient toproduce on decomposition of the tetra-alkyl-orthosilicate a coating ofsilica having an optical thickness of an odd multiple of a quarter wavelength of the incident light, evaporating the solvent from said liquidlayer so as to bring about the decomposition of thetetra-alkyl-orthosilicate by the acid on the surface of the article andthe production of the silica coating in situ on said surface and to thedesired optical thickness, said water soluble substance remaining stablethrough said evaporation of the solvent and being minutely dispersedthrough the silica coating produced, and washing said coated articlewith water so as to dissolve the water soluble substance and 12 removesaid substance from the coating thereby increasing its porosity.

7. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of controlled amounts oftetra-alkyl-orthosilicate selected from the group consisting oftetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof,a mineral acid inert to silica as a decomposing agent for thetetra-alkyl-orthosilicate and a relatively involatile coloring materialsubstantially inert in the presence of the acid, thetetra-alkyl-orthosilicate, the solvent to be used and the material ofthe article, the tetra-alkylorthosilicate being decomposable by saidacid to produce silica, said coloring material, tetraalkyl-orthosilicateand acid being dissolved in a volatile organic solvent miscible witheach, said solvent being of the group selected from mono ethyl ether ofethylene glycol, mono butyl ether of ethylene glycol, ethyl acetate,methyl acetate, amyl acetate, ethyl alcohol, methyl alcohol, isopropylalcohol, butyl alcohol, and mixtures thereof, the proportion of acidembodying approximately 1 to 10% of the solution, the combinedproportions of acid and solvent embodying at least of the solution, theproportion of tetra-alkyl-orthosilicate embodying approximately 1 to 10%of the solution and the coloring material embodying substantially thebalance of the solution, the tetra-alkyl-orthosilicate being held in thesolution in a substantially undecomposed state until the concentrationof the tetraalkyl-orthosilicate and the acid is increased by evaporationof the solvent sufliciently to permit the acid to decompose thetetra-alkyl-orthosilicate, causing the liquid solution to form on saidsurface of the article a liquid layer of a uniform thickness which willcontain an amount of tetra-alkyl-orthosilicate and acid sufficient toproduce on decomposition of the tetra-alkylorthosilicate a coating ofsilica having an optical thickness of an odd multiple of a quarter wavelength of the incident light, and evaporating the solvent from saidliquid layer so as to bring about the decomposition of thetetra-alkylorthosilicate by the acid on the surface of the article andthe production of the silica coating in situ on said surface and to thedesired optical thickness, said coloring material remaining stablethrough said evaporation and being dispersed through the resultantsilica coating.

8. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of controlled amounts oftetra-alkyl-orthosilicate selected from the group consisting oftetraethylorthosilicate, tetramethylorthosilicate and mixtures thereof,a mineral acid inert to silica as a decomposing agent for thetetra-alkyl-orthosilicate and a relatively involatile materialsubstantially inert in the presence of the acid, thetetra-alkyl-orthosilicate, the solvent to be used and the material ofthe article, said inert material when dispersed in the resultant coatinghaving light transmission altering characteristics other than thatpossessed by silica, the tetra-alkyl-orthosilicate being decomposable bysaid acid to produce silica, said light altering material,tetra-alkyl-orthosilicate and acid being dissolved in a volatile organicsolvent miscible with each, said solvent being of the group selectedfrom mono ethyl ether of ethylene glycol, mono butyl ether of ethyleneglycol, ethyl acetate,

QQTMSOQH methyl acetate. amyl acetate, ethyl alcohol, methyl alcohol,isopropyl alcohol, butyl alcohol; and mixtures thereof. the proportionof acid embodying approximately 1 to 10% of the solution. the combinedproportions of acid and solvent embodying at least 90% of the solution,the propor tion of tetra-alkyl-orthosilicate embodying ap proximately 1to 10% of the solution and the light altering material embodyingsubstantially the balance of the solution, the tetra-alkyl-orthosilicatebeing held in the solution in a substantially undecomposed state untilthe concentration of the tetra-alkyl-orthosilicate and the acid isincreased by evaporation of the solvent sufficiently to permit the acidto decompose the tetra-alkyl-orthosilicate, causing the liquid solutionto form on said surface of the article a liquid layer of a uniformthickness which will contain an amount of tetra-alky .orthosilicate andacid sufficient to produce on decomposition of thetetra-alkyl-orthosilicate a coating of silica having an opticalthickness of an odd multiple of a quarter wave length of the incidentlight, and evaporating the solvent from said liquid layer so as to bringabout the decomposition of the tetra-alkylorthosilicate by the acid onthe surface ofthe article and the production of the silica coating insitu on said surface and to the desired optical thickness, said lightaltering material remaining stable through said evaporation and beingdispersed through the resultant silica coating.

9. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of by volume approximately 7% oftetra-ethyl-orthosilicate, approximately 1% hydrochloric acid,decomposing agent for the tetra-ethyl-orthosilicate, thetetra-ethylorthosilicate being decomposable by said acid to producesilica, approximately 20% of the mono ethyl ether of ethylene glycol andapproximately 72% denatured alcohols, said tetra-ethyl-orthosilicate andacid being dissolved in said organic solvents, thetetra-ethyl-orthosi1icate being held in the solution in a substantiallyundecomposed state until the concentration of thetetra-ethylorthosilicate and the acid is increased by evaporation of thesolvents sufiiciently to permit the acid to decompose thetetra-ethyl-orthosilicate, causing the liquid solution to form on saidsurface of the article a liquid layer of a uniform thickness which willcontain an amount of tetraethyl-orthosilicate and acid sufficient toproduce on decomposition of the tetra-ethyl-orthosilicate a coating ofsilica having an optical thickness of an odd multiple of a quarter wavelength of the incident light, and evaporating the solvents from saidliquid layer so as to bring about the decomposition of thetetra-ethyl-orthosilicate by the acid on the surface of the article andthe produc tion of the silica coating in situ on said surface and to thedesired optical thickness.

10. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting of by volume approximately oftetra-ethyl-orthosilicate, approximately 5% of concentrated hydrochloricacid as a decomposing agent for the tetra-ethyl-orthosilicate, thetetra-ethyl-orthosilicate being decomposable by said acid to producesilica, approximately 24% ethylene glycol mono ethyl ether,approximately 10% ethylene glycol mono butyl ether, approximately 16%butyl alcohol and approximately 40% denatured alcohol, said tetra-SEARCH m 14 ethyl-orthosilicate and acid being dissolved in said organicsolvent, the tetra-ethyl-orthosilicate being held in the solution in asubstantially undecomposed state until the concentration of thetetra-ethyl-orthosilicate and the acid is increased by evaporation ofthe solvent sufficiently to permit the acid to decompose thetetra-ethyl-orthosilicate, causing the liquid solution to form on saidsurface of the article a liquid layer of a uniform thickness which willcontain an amount of tetraethyl-orthosilicate and acid sufiicient toproduce on decomposition of the tetra-ethyl-orthosilicate a coating ofsilica having an optical thickness of an odd multiple of a quarter wavelength of the incident light, and evaporating the solvent froni saidliquid layer so as to bring about the decomposition of thetetra-ethyl-orthosilicate by the acid on the surface of the article andthe production of the silica coating in situ on said surface and to thedesired optical thickness.

11. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting by volume of approximately 1 to 10% oftetra-ethyl-orthosilicate and approximately 1 to 10% of a mineral acidinert to silica as a decomposing agent for the tetra-ethylorthosilicate,the tetra-ethyl-orthosilicate being decomposable by said acid to producesilica, said tetra-ethyl-orthosilicate and acid being dissolved in themono ethyl ether of ethylene glycol, said mono ethyl ether of ethyleneglycol embodying substantially the balance of the solution, thetetra-ethyl-orthosilicate being held in the solution in a substantiallyundecomposed state until the concentration of thetetra-ethyl-orthosilicate and the acid is increased by evaporation ofthe mono ethyl ether of ethylene glycol from the solution sufiicientlyto permit the acid to decompose the tetra-ethyl-orthosilicate, causingthe liquid solution to form on said surface of the article a liquidlayer of a uniform thickness which will contain an amount oftetra-ethyl-orthosilicate and acid sufficient to produce ondecomposition of the tetra-ethyl-orthosilicate a coating of silicahaving an optical thickness of an odd multiple of a quarter wave lengthof the incident light, and evaporating the mono ethyl ether of ethyleneglycol from said liquid layer so as to bring about the decomposition ofthe tetra-ethyl-orthosilicate by the acid on the surface of the articleand the production of the silica coating in situ on said surface and tothe desired optical thickness.

12. The method of producing a surface reflection reducing coating on thesurface of an article comprising the steps of applying to said surface aliquid solution consisting by volume of approximately 5%tetra-ethyl-orthosilicate and approximately 1% of a mineral acid inertto silica as a decomposing agent for the tetra-ethyl-orthosilicate, thetetra-ethyl-orthosilicate being decomposable by said acid to producesilica, said tetra-ethyl-orthosilicate and acid being dissolved in themono ethyl ether of ethylene glycol, said mono ethyl ether of ethyleneglycol embodying approximately 94% of the solution, thetetraethyl-orthosilicate being held in the solution in a substantiallyundecomposed state until the concentration of thetetra-ethyl-orthosilicate and the acid is increased by evaporation ofthe mono ethyl ether of ethylene glycol from the solution sufficientlyto permit the acid to decompose the tetra-ethyl-orthosilicate, causingthe liquid solution to form on said surface of the article a liquidlayer of a uniform thickness which will contain an amount oftetra-ethyl-orthosilicate and acid suflicient to produce ondecomposition of the tetra-ethyl-orthosilicate a coating of silicahaving an optical thickness of an odd multiple of a quarter wave lengthof the incident light, and evaporating the mono-ethyl ether of ethyleneglycol from said liquid layer so as to bring about the decomposition ofthe tetra-ethyl-orthosilicate by the acid on the surface of the articleand the production of the silica coating in situ on said surface and tothe desired optical thickness.

13. An article of the character described comprising a piece of of lighttransmitting material having a thin transparent surface reflectionreducing coating of substantially pure silica formed on a surfacethereof, said coating being microporous and consisting of the driedsilica product of tetramethyl-orthosilicate hydrolyzed in situ by anacid inert to silica and the light transmitting material, said coatinghaving a substantially uniform optical thickness of an odd multiple of aquarter wave length of the incident light and a substantially uniformeffective index of refraction less than that of massive silica, saidcoating being firmly adherent to the piece of light transmittingmaterial and being resistant to abrasion.

14. An article of the character described comprising a substrate havinga thin transparent surface reflection reducing coating formed on asurface thereof, said coating being micro-porous and consisting of thedried silica product of a tetraalkyl-orthosilicate selected from thegroup consisting of tetra-ethyl-orthosilicate, tetra-methyl- 16orthosilicate and mixtures thereof hydrolized in situ by an acid inertto silica and the substrate. and containing inert coloring substancesdispersed therethrough, said coating having a substantially uniformoptical thickness of an odd multiple of a quarter wave length of theincident light and a substantially uniform effective index of refractionless than that of massive silica, said coating being firmly adherent tothe substrate and being resistant to abrasion.

HAROLD R. MOULTON.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,809,755 King et al June 9, 19312,118,898 Price May 31, 1938 2,220,862 Blodgett Nov. 5, 1940 2,329,632Marsden Sept. 14, 1943 2,347,733 Christensen May 2, 1944 2,356,553Weissenberg Aug. 22, 1944 2,366,516 Geficken et al Jan. 2, 19452,371,611 Dimmick Mar. 20, 1945 2,384,209 Sukumlyn Sept. 14, 19452,428,357 Cohen Oct. 7, 1947 2,432,484 Moulton Dec. 9, 1947 FOREIGNPATENTS Number Country Date 538,273 Great Britain 1941

